The present invention relates to improved composites of the type containing a fiber network and a polymer matrix, and especially to such composite materials wherein both the fiber and polymer are polyolefins.
Composite materials containing fibers and polymer matrix are frequently employed to increase the physical properties (e.g. tensile strength and modulus) of the polymer. Such materials may be in the form of sheets, films, molded objects of various shapes, stamped parts, shaped extrusions and pultrusions, foams and sandwich constructions. The fibers frequently employed are glass, graphite fibers, boron, aramids (wholly aromatic polyamides) and poly(ester terephthalate). While such fibers have excellent mechanical properties and high melting points and softening temperatures, composites prepared therefrom do not have a gain in properties that might be expected by the rule of mixtures. This reduction in gain may be attributable to weaknesses at the fiber-polymer interface or other factors.
Mead & Porter, J. Appl. Polymer Sci., vol. 22, pp. 3249-3265 (1978) report composites prepared from solid state extruded low molecular weight (58,000 or 18,400 number average molecular weight) polyethylene fibers and polyethylene matrices (both high and low density). In preparing the composites, bonding occurs at sufficient temperatures to bond the matrix to the fibers, which may also be sufficient to reduce the crystallinity or order of the fibers, at least at or near the fiber-matrix interface. FIG. 8 of the article presents the theoretical tensile strengths of the composites as a function of the volume fraction of the fiber. The actual results achieved were 0.025 GPa at 0.4 volume fraction fibers (about 50% of theory) and 0.062 GPa at 0.7 volume fraction fibers (about 65% of theory)